Investment Castings – Rapid Designs Provides a “Rapid Prototyping Bridge to Production” Metal Casting Solution
Rapid Designs offers expert advice as well as attentive service in the production and utilization of investment castings as part of its fully-integrated product development and rapid manufacturing services.
Rapid Designs team of specialists provides special expertise in investment castings, based on the company's origins 40 years ago as a model and pattern maker for the foundry industry. Today this experience provides Rapid Designs customers with access to the highest quality makers of investment castings as well as expert advice on the appropriateness of investment castings versus other approaches in the spectrum of product development processes.
Investment Casting Process – Overview
Prototype and low quantities of metal investment castings are typically produced from a tooling master and specialty wax patterns. The process can yield production accuracies of +/- .005 inch/inch (.13 mm). Production molds used to manufacture metal investment castings are commonly made using either ceramic or plaster. The ceramic mold is fired in a furnace to drive out moisture. In both processes the wax is liquefied and burns off.
Investment castings are well suited for both long and short runs, and are typically recommended for applications in which fine detail, design complexity and accuracy are required.
The metal casting manufacturing process is also known as lost wax. Today it is often used to rapid prototype metal parts. The investment casting process begins with an investment casting pattern typically made of custom-formulated foundry wax. Wax as a pattern media is a used universally throughout the investment casting and jewelry industry. Quality formulations of wax provide a stable and accurate pattern media. A high quality wax will leave no ash after burn out to contaminate the metal casting. When fired the wax pattern will liquefy and completely melt out of the mold leaving a hollowed out form representing the final part geometry. Molten metal is poured (cast) into the mold, forming the final part. The metal part is then ready for machining and finishing operations.
Plaster Mold Investment Casting Process
This is the best process to simulate aluminum or zinc die cast parts in both properties and appearance. Aluminum or zinc parts can be shipped in as little as 2-5 weeks from receipt of 3D CAD part geometry, depending on size and quantity.
The rubber plaster mold process offers an alternative to ceramic investment casting, sand casting and prototype die casting. This process is both suitable for short run production quantities of aluminum and zinc parts.
Castings from plaster molds simulate die cast parts for appearance and properties. Tooling is low cost and easily accommodates geometry modifications and design enhancements. The rubber plaster molding process also serves as a short run (<2,000 /year) production process for programs that cannot justify the cost of hard tooling.
Castings from the rubber plaster mold process feature excellent surface finishes, complex geometry (including undercuts and complex coring).
Investment casters who use the plaster mold process do not require a wax patterns. Instead, the plaster mold process begins by using a pattern master. Today the pattern master is produced using rapid prototype (RP) technologies. Typically an SLA (stereolithography) model is made of the actual part using “as cast” CAD geometry. This master is used to produce a temporary rubber negative which, in turn is used to produce a final rubber positive. Plaster slurry is then poured over the rubber positive to produce the plaster mold. The foundry then pours the metal into the plaster mold. Once the metal solidifies the casting is removed from the mold by breaking the plaster away from around the part and consequently destroying the mold. As with all metal castings machining stock is added to areas requiring close tolerances not achievable in the “as cast” form.
Wax patterns can be manufactured using a variety of techniques. In production wax injection molds are machined from billet aluminum blocks. Single cavity molds are produced for low-volume production runs. Multiple-cavity molds are made for high-volume production quantities. Wax is then injected into the machined aluminum molds to create multiple investment casting wax patterns. Machined molds are time-consuming and costly if you only need development engineering prototypes to prove out and verify your design, or low volume quantities.
Ceramic Mold Investment Casting Process
Several rapid prototyping technologies and processes have been devised to provide rapid cost effective solutions for prototyping and low volume cast metal alloy parts. The most commonly used rapid prototyping processes and materials are SLS CastForm™ and SLA QuickCast™. Both processes are direct and require no intermediate mold. These are the investment casting patterns. They are directly invested and used to make the ceramic mold. For every metal casting, one investment casting pattern is required.
As described above when the ceramic mold is fired, the pattern is burned off. While both of these RP processes are widely used, Rapid Designs frequently utilizes the SLS CastForm process. Investment casting suppliers that we use prefer CastForm because it leaves less ash content after firing.
SLS CastForm™ pattern material is a composite comprising of ~50% polystyrene plastic (sintered density) and 50% formulated foundry wax. After processing, the SLS CastForm polystyrene pattern it is then dipped in foundry wax to complete the investment casting pattern making process.
CastForm before wax dipping »
After wax dipping »
Aluminum invested castings
SLA QuickCast™ is made from a UV curable polymer (epoxy resin). The quick cast is a patented build style. The build style creates a honeycomb configuration within the part wall thickness. The process acts to minimize the epoxy resin mass in the pattern thus optimizing burn-out and minimizing ash residue. It further serves to prevent the pattern from expanding during the ceramic firing causing the ceramic to crack prematurely.
Rapid Designs applies two other processes to make investment casting patterns. Where highly accurate and intricately detailed castings are required, Rapid Designs proprietary AccuWax™ pattern process creates accurate wax patterns using cost-effective soft tooling. A master model is created using one of several options: SLA RP, machining or hand made (carved or sculpted), whichever is best suited to achieve the part geometry and the customer's desired results. In this process, the master model is used to create a silicone rubber mold (SRM) or a soft machined mold is created. Using our specially formulated foundry wax, it is cast into the mold to create the investment casting patterns. A high degree of accuracy is attainable using this process but often multiple master models and soft molds may be needed to zero in on the desired part as-cast accuracy.
Image shows a knee implant. Process demonstrates (left to right):
»master model » wax pattern » Titanium casting
Our Suppliers – The Critical Link to Our Success
Rapid Designs partners with metal investment casters who employ the ceramic mold process and the plaster mold process. Our partner network understands the rapid business model and has adopted Rapid Designs-like standards. Each customer application is carefully reviewed by Rapid Designs technical specialists to determine the best process. A decision matrix considers part geometry, specified metal alloy, fine feature details, as-cast required accuracy, as-cast surface finish and quantity of metal investment castings needed.
Investment castings are widely used in military air frame applications in addition to a wide range of commercial products where precision is vital, including medical implants, jewelry and larger industrial components.
With broad experience in all aspects of rapid prototyping, rapid tooling and rapid manufacturing direct from digital input, Rapid Designs is ideally suited to assisting product development specialists in the application of investment castings.
Investment Casting – Products & Services
- SLS CastForm™ investment casting patterns
- AccuWax™ Wax investment casting patterns
- Investment casting – ceramic process
- Investment casting – plaster process
- Precision Machining to client supplied drawings and specifications
- Metal post processing and finishing
- Reverse Engineering of legacy parts
Investment Casting – Materials & Casting Sizes
- Ceramic Process (1/2 oz. To 225 lbs.):
- Aluminum A356
- Steel Alloys
- Stainless Steel 300 and 400 series
- Ductile Iron
- Plaster Process (1/2 oz. To 225 lbs.):
- Aluminum A356
Contact Rapid Designs – Your Prototype Model-making Experts
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